The present invention relates to a preload measuring apparatus of a rolling bearing which is assembled into each type of precision rotational portions such as a spindle motor, rotary actuator, rotary encoder, or the like, of a video tape recorder (VTR), hard disk drive (HDD), laser beam printer (LBP), or similar device.
A rolling bearing which is assembled into a precision rotational portion such as a spindle motor, rotary actuator, rotary encoder, or the like, used in a VTR, HDD, LBP, or similar device, is required to be very accurately produced to prevent a whirling motion and a deflection in the axial direction. Accordingly, a rolling bearing to support the spindle is used under the condition that a preload in the axial direction is applied. By applying the preload, the rigidity of the bearing is kept high, the deflection accuracy of the shaft is increased, and the slippage of a ball at the high speed rotation is prevented.
As such the rolling bearing to support the spindle of the VTR, HDD, LBP, or the like, there are following rolling bearings: for example, as shown in FIG. 11(a), a pair of respectively independent rolling bearings assembled onto a rotation shaft by press fitting or adhesion; as shown in FIG. 11 (b), a rolling bearing which has double-row raceways in the outer ring, and to which the preload is applied by press fitting or adhering the divided inner ring to be moveable in the axial direction; or as shown in FIG. 11 (c), a rolling bearing which has double-row raceways in the inner ring and the outer ring respectively, and to which the preload is applied at assembling by changing the dimension of the pitch of the raceway.
In the other words, in the rolling bearing shown in FIG. 11 (a), an inner race member thereof comprises a shaft and two inner rings attached to the shaft, and an outer race member thereof comprises two outer rings which are respectively mated with the inner rings. In the rolling bearing shown in FIG. 11(b), an inner race member comprises a shaft and two inner rings attached to the shaft, and an outer race member thereof comprises a common outer ring having two raceways which are respectively mated with the two inner rings. In the rolling bearing shown in FIG. 11(c), an inner race member thereof comprises a shaft having two raceways, and an outer race member thereof comprises an outer ring having two raceways which are respectively mated with the two raceways of the shaft.
However, in any method of the above description, it is difficult to measure the preload after the assembling of the rolling bearing, because the preload is different according to the positional dimension of each member.
Accordingly, for example, Japanese Patent Examined Publication No. Hei. 2-61100 disclose a preload measuring method wherein the resonance frequency of the bearing apparatus itself is obtained by applying a minute vibration onto the rolling bearing, and this resonance frequency is converted into the preload by using a constant relationship between the resonance frequency of the rolling bearing and the preload.
However, in such the preload measuring method, there is still a problem which will be described below. That is, noise vibration is generated due to a disturbance of the mechanical system when a minute vibration is applied onto the rolling bearing, and therefore the preload can not be accurately measured due to this noise vibration.
Further, as the rolling bearing becomes small and then the mass of the outer ring becomes small, the resonance frequency of the rolling bearing appears on the high frequency side. As shown in FIG. 7, the vibration peak appears, for example, near 20 kHz at which the noise is relatively strong. At that time the resonance frequency is covered with the noise, so that it becomes difficult to detect the resonance frequency. Accordingly, the accuracy of the measurement is lowered, and the rigidity or the preload of the rolling bearing which are calculated, becomes incorrect.